Cleaving the N,N Triple Bond: The Transformation of Dinitrogen to Ammonia by Nitrogenases - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 165

The P-cluster of the VFe protein has long been thought to closely resemble the
P-cluster of the MoFe protein in structure, but the recent characterization of
the A. vinelandii VFe protein suggested otherwise [ 22 , 92 ]. The EPR data of the
dithionite-reduced and IDS-oxidized forms of the VFe protein strongly indicate that
the electronic properties of the P-cluster in the VFe protein are quite different
than those of its counterpart in the MoFe protein [ 92 ]. This observation is consistent
with the outcome of the Fe K-edge EXAFS analysis of a cofactor-deficient form of
the VFe protein, which suggests that this protein carries a P-cluster species that
consists of a pair of [Fe 4 S 4 ]-like clusters (Figure 8b )[ 93 ]. Interestingly, a highly
similar conformation has also been observed earlier in the case of the MoFe protein,
where a precursor form to the P-cluster was identified as a pair of [Fe 4 S 4 ]-like
clusters [ 94 ].
The iron-vanadium cofactor (FeVco) of the VFe protein has been characterized
in the protein-bound and solvent-extracted states by a number of spectroscopic
methods, including EPR, XAS/EXAFS, and M¨ssbauer spectroscopy [ 95 - 101 ].
Results from the Fe K-edge XAS/EXAFS studies suggest that the FeVco is struc-
turally similar to the FeMoco except for the presence of a different heterometal in
the cofactor [ 95 - 98 ]. Further, these studies reveal that the isolated FeVco adopts an
overall octahedral geometry, whereas the isolated FeMoco assumes an overall
tetrahedral/trigonal pyramidal structure. This observation suggests that the FeS
core of the FeVco, albeit structurally similar to the FeS core of the FeMoco, is
slightly elongated compared to the latter (Figure 8b ). This argument is also
supported by the observation of a longer Fe-V backscattering path in FeVco
(2.88 )[ 101 ].

Consistent with the outcome of the XAS/EXAFS studies, EPR analysis
reveals differences in the electronic properties of the two cofactors. The isolated
FeVco displays an S

Å ) than the Fe-Mo backscattering path in FeMoco (2.68 Å

3/2 signal that is similar to, yet significantly broader in
line-shape, than that of the isolated FeMoco, suggesting a modulating effect of the
electronic properties of these cofactors by the presence of different heterometals
[ 101 ]. Other than the information gathered on the core structure of the FeVco,
there is currently no direct evidence proving homocitrate as the organic moiety
in the FeVco, nor is there conclusive data demonstrating the presence of an
interstitial carbide in this cofactor. However, given the similarity between the
structural and biochemical properties of the two cofactors, it is assumed that a
homocitrate entity (or one of its analogs) can be found along with a central carbide
in the structure of the FeVco.

¼

4.2 The Catalytic Features

4.2.1 The Reduction of Dinitrogen

While the V-nitrogenase resembles the Mo-nitrogenase in catalysis, it exhibits
some catalytic features that are notably distinct from those of its Mo-counterpart.